Method of diffusion of molybdenum into extrouded ferrous metal surfaces of a piston



g- 13, 1953 E. F. CHRISTIANSEN 3,396,447

METHOD OF DIFFUSION OF MOLYBDENUM INTO EXTRUDED FERROUS METAL SURFACESOF A PISTON Filed Jan. 19. 1966 United States Patent 3,396,447 METHOD OFDIFFUSION 0F MOLYBDENUM INTO EXTRUDED FERROUS METAL SURFACES OF A PISTONErik Frede Christiansen, Sonderborg, Denmark, as- 5 signor to Danfoss A/S, Patent Department, Nordborg, Denmark Filed Jan. 19, 1966, Ser. No.521,688 Claims priority, application (gzrmany, Jan. 30, 1965,

7 Claims. c1: 29-1565) 10 ABSTRACT OF THE DISCLOSURE This inventionrelates to a method for introducing molybdenum into a ferrous metalsurface and to articles having such a surface into which molybdenum wasintroduced by the method of the invention. According to the invention,molybdenum is caused to diffuse into a surface of a ferrous metalarticle. Thus, the invention also relates to ferrous metal articleshaving molybdenum diffused into a surface thereof.

The expression ferrous metal is intended to denote iron and alloys ofiron such as steels.

The primary object of introducing molybdenum into a ferrous metalsurface according to the invention is to decrease the coefficient offriction .of the surface. Thus, the invention is particularly usefulwhere the ferrous metal surface is a surface of a machine part orelement which slides along another machine part or element during theoperation of the machine. Thus, for example, a surface which is treatedaccording to the invention may be the peripheral surface of a pistonwhich reciprocates in a cylinder, as for instance a piston for acompressor of a refrigerating unit.

It is known to provide running surfaces (i.e., surfaces which slidinglyengage other surfaces) with molybdenum sulfide in order to decrease thecoefficient of friction of the running surfaces. For example, molybdenumsulfide powder is rubbed into the surface or molybdenum trioxide iselectrolytically deposited on the surface and is subsequently convertedto molybdenum sulfide by expo- 5 sure to a hydrogen sulfide atmosphere.However, the effective lives of these treatments are limited. Thus, forexample, the effect of such treatments does not last long enough for usein hermetically capsuled small refrigerating units, which are providedwith lubrication for their complete guarantee period of operation, fiveyears.

According to the invention, it has been found that molybdenum willdiffuse under the influence of elevated temperature into a ferrous metalsurface which has been formed by extrusion. Apparently, a crystalline oratomic orientation occurs at such a surface, which orientation permitsunder the influence of elevated temperature the diffusion of molybdenuminto the surface. When an article is formed by extrusion, surfaces ofthe article are attenuated and/or are subjected to shear by the walls ofthe extrusion die, the attenuation and shear occurring notably in thedirection of extrusion. Accordingly, the

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above referred to orientation will be in the general direction ofextrusion. Furthermore, the more attenuated or subjected to shear that agiven surface is by the extrusion operation, the more greatly orientatedand therefore receptive to diffusion by molybdenum will the surface be.

By practicing the invention one can readily produce a molybdenum layerof for example 25 microns thickness on an extruded ferrous metalsurface. This is merelyexemplary. Such a layer can be readily achievedfor example by the use of pure molybdenum powder, and in a smallrefrigerating unit piston, for example, Will last for well over fiveyears. The method of this invention is not to be confused with the knownmethod in which the major purpose of molybdenum sulfide powder is tofill irregularities in the surface. In the present invention, there isactual diffusion of molybdenum into inter-atomic or inter-crystallineinterstices. It is found that ferrous surfaces having low carboncontents are especially suitable for the method of the invention. Thus,such surfaces are iron itself or low carbon steels. However, theinvention is operative with high carbon steels also.

The method of the invention is not to be confused with the alloying oftin with molybdenum in order to obtain better wearing properties andsubsequently extruding the alloy. The amount of molybdenum used in thisalloy is typically one half to one percent. Such alloy is harder thantin itself and is therefore less suitable for extrusion. Furthermore, itis found that the Wearing properties are not significantly improved asthe concentration of molybdenum is too small. Also, such an alloy doesnot manifest the friction reducing properties of molybdenum.

The exact temperature used for the heat treatment is not critical. Ineach instance, the temperature is elevated sufficiently to cause themolybdenum to diffuse into the extruded ferrous surface. Merely as anexample, temperatures between about 1000 C. and 1200 C. are found to bevery suitable. In order to prevent oxidation of the extruded ferroussurface or of the molybdenum during the heat treatment, it is frequentlydesirable to conduct the heat treatment in a reducing atmosphere such ashydrogen or in a protective atmosphere of inert gases such as nitrogenor the so-called rare or precious gases.

Generally, the best results are attained when molybdenum is used alone.However, it is not necessary to use the molybdenum alone. Accordingly,for example, the molybdenum may be used in admixture with manganese.

The molybdenum or molybdenum-containing powder may be kept in contactwith the extruded ferrous surface during the heat treatment by beingallowed to rest on the surface or a substance may be used to adhere thepowder to the surface. The substance may for example be petroleum oil, asynthetic resin such as an epoxy resin, a natural resin, or any othermaterial which will adhere the powder to the surface.

The method according to the invention of treating an extruded ferrousmetal surface may be carried out simultaneously with other treating orfabricating operations. Thus, for example, an article having a surfaceto be treated by the invention may be joined with solder to anotherarticle and a subsequent heating may accordingly be used not only to aidthe diffusion of molybdenum into the surface but also to complete thesoldering by fusing the solder. In this instance, a soldering oven wouldsimultaneously bring about both the diffusion of the molybdenum into thesurface and the completion of the soldering of the joint.

The invention will now be further described by reference to a specificembodiment thereof in connection with the drawing. While the inventionwill be described as applied to a piston it will be understood that itis applicable to other elements and devices having extruded ferroussurfaces which are to have their frictional and wear characteristicsgreatly improved.

A piston 1 is extruded from a sheet of low-carbon steel into a pot-likeconfiguration having a bottom and a skirt or extension wall 6. By asubsequent operation, the free end 7 of the skirt 6 is formed inward tocome into good contact with a crankshaft linkage element or scotch-yoke3 to which it is connected by solder spots 2. In a central opening 4 ofthe crankshaft linkage element 3 is disposed the crank pin of acrankshaft of an engine (not shown). By this construction the piston 1is reciprocated axially in a cylinder (not shown).

Molybdenum powder, or molybdenum trioxide powder, or a mixture ofmolybdenum and manganese powders (for example in the ratio 60:40respectively) is mixed with petroleum oil or an epoxy resin as anadhering medium and is applied to the outside surfaces of the piston 1as a layer. The joined piston 1 and crankshaft linkage element 3 arethen placed in a conventional soldering oven which is heated to atemperature of about 1100' C. This heating simultaneously causes thesolder spots 2 to fuse and thereby complete the soldering and alsocauses the powder to diffuse into the outside surfaces of the piston 1.

Actually, only the outside surface of the skirt 6 requires the treatmentof the invention, as only this surface is subjected to frictionaloperation. The method of the invention is particularly adapted fortreatment of the outer surface of the skirt 6, because the skirt 6 hasbeen particularly greatly attenuated and its outer surface subjected toshear by the extrusion by which the piston 1 was formed and,accordingly, the outer surface of the skirt 6 is particularlysusceptible to diffusion by molybdenum. This will be appreciated byconsidering that the sheet of steel which is extruded to form the piston1 is extruded in a direction parallel or essentially parallel to theaxis of the piston 1. The sheet of steel is initially of uniformthickness and the fact that the skirt 6 is especially greatly attenuatedcan be appreciated by noting that the thickness of the skirt 6 issubstantially less than the thickness of the bottom 5. Since the outersurface of the bottom 5 does not require the treatment it is notnecessary that this part of the outer surfaces of the piston 1 beprovided with the layer of powder; however, in a mass productionoperation it may be more convenient simply to provide the entire outersurfaces of the piston 1 with a layer of powder rather than to segregatethe outer surface of the bottom 5.

A soldering oven is normally provided with an atmosphere of protectingor reducing gas and, as noted above, this is also desirable for thetreatment of the invention.

The molybdenum or molybdenum-containing powder may conveniently beapplied in a mass production operation by holding the article having thesurface to be treated in a fluidized bed of the powder. If desired, anadhering medium may be intermixed with the powder in the fluidized bed.Alternatively, molybdenum trioxide may be electrolytically deposited onthe surface and a reducing (e.g., hydrogen) atmosphere may be providedfor the heat treatment, whereby molybdenum will deffuse into thesurface.

The invention is not to be construed as limited to the particular formsdisclosed herein, since these are to be regarded as illustrative ratherthan restrictive.

What I claim and desire to secure by Letters Patent is:

1. A method for introducing molybdenum into a ferrous metal surface of apiston comprising forming said piston including said surface byextrusion in a direction essentially parallel to the axis of the pistonwith said surface extending in a direction generally parallel to saidaxis, whereby said surface is more highly attenuated by said extrusionthan other surfaces of the piston, contacting said surface withmolybdenum, and supplying heat to said surface in contact with themolybdenum to cause diffusion of the molybdenum into said surface.

2. A method according to claim 1, wherein the heating is carried out ina protective gas atmosphere.

3. A method according to claim 1, wherein the protective gas atmosphereis a reducing atmosphere.

4. A method according to claim 1, wherein said surface is also contactedwith manganese and the heat is applied while said surface is in contactboth with the manganese and the molybdenum.

5. A method according to claim 1, wherein the molybdenum is held incontact with said surface with an adhesive.

6. A method according to claim 1, wherein said piston is joined withsolder to an article and heat is supplied to the solder simultaneouslywith the heating of said surface, whereby in a single operation themolybdenum is caused to diffuse into said surface and the solder isfused to complete the joining with solder.

7. A method according to claim 1, wherein the piston is joined withsolder to a crankshaft linkage element, before said surface is suppliedwith heat while in contact with the molybdenum.

References Cited UNITED STATES PATENTS 1,717,750 6/1929 Wills 29156.52,201,405 5/ 1940 McCarroll 92223 2,410,405 11/ 1946 Cornelius 92223 X2,817,562 12/1957 Fleming et al 29-156.5 2,833,264 5/1958 Dailey et al.29l56.5 2,833,668 5/1958 Dailey et a1 29-156.5 2,984,895 5/1961 Grifiinet al 29l56.5 X 3,041,194 6/1962 Rosen et a1. 29-156.5

FOREIGN PATENTS 1,037,220 8/ 1958 Germany.

JOHN F. CAMPBELL, Primary Examiner. P. M. COHEN, Assistant Examiner.

